Many chemical manufacturing processes employ chemical reactors to convert chemical feedstocks to desired solid, gaseous or liquid intermediates and end products. During the production of these desired materials, the materials pass through a wide variety of process equipment located downstream of the chemical reactor. This process equipment typically conveys, treats, reacts or otherwise operates on the intermediates, waste or recycle streams, or end products so that the desired end product is produced in the desired form for shipment to the end user.
An example of one such process is the gas phase production of polyolefins such as polypropylene or polyethylene from gaseous feedstocks. In this type of process, one or more gaseous monomer feedstocks reacts in the chemical reactor in the presence of a catalyst to produce a powdered polymer. The powdered polymer typically then is extruded in a finishing unit to produce pellets. These pellets are easy to ship, and are used to fabricate polymer-based manufactured products such as molded articles, films and fibers.
Under ideal circumstances, reactors such as the polymerization reactors discussed above will produce powdered product having a relatively predictable distribution of particle sizes. Downstream powder handling and finishing equipment is designed to accommodate this predicted distribution of particle sizes. Unfortunately, under less than ideal conditions, the polymerization reactor can produce xe2x80x9clumpsxe2x80x9d or xe2x80x9cstringsxe2x80x9d, which are agglomerates of polymer particles having a significantly greater size than expected.
The presence of strings and lumps can serve as an early indication that the polymerization reactor is not functioning optimally. Unfortunately, unless reactor performance is seriously degraded, the presence of strings and lumps often can not be observed directly. Additionally, the presence of strings and lumps in polymer powder is often masked by the pelletization process, in which the extruder operates under temperature and pressure conditions sufficient to pelltize some undesirably large particles.
Sub-optimal reactor performance as indicated by the presence of undesired large particles typically requires a change in reactor operating parameters to improve operations. Furthermore, while the product finishing process can, at times, force strings and lumps to take a macroscopic form desired by the end user (such as that of a pellet), the delivered material may contain substantial sub-macroscopic inhomogeneous regions (i.e. localized regions within the pellet) which may affect use of the delivered material by the end user.
In many cases, problems such as those noted above can be detected prior to delivery of product to a customer through various sampling and quality control programs. The use of such programs cannot, however, provide early, real time indication of sub-optimal process conditions that would enable the reactor operator to take corrective action to avoid or minimize the formation of inhomogeneous material.
On-line product analyzers can be used to provide information useful for process control to minimize product inhomogeniety in some cases, but on-line analyzers are not available to detect many types of undesirable product variations.
What is needed is a sensitive, real time method for indirectly monitoring the formation of intermediate and product inhomogeneity in a chemical manufacturing process, without having to resort to the addition of on-line product analysis equipment. The method should allow the chemical manufacturing process operator to intervene, manually or through automated control systems, to minimize or eliminate formation of inhomgeneous material. Such early detection and intervention would enable correction of process parameters in equipment such as the chemical reactor before major upsets in the chemical reactor or other equipment occurs, and before substantial amounts of off specification material are manufactured.
Surprisingly, we have found that by monitoring signals from downstream process equipment for subtle transient responses, we can detect the presence of nonhomogeneous products such as polymer strings, lumps and other abnormally large particles. We then use that information to take corrective action before the presence of these undesired products becomes pronounced enough to cause upsets in process equipment, and at a time earlier than would be apparent from direct observation of reactor or other upstream equipment operating parameters or from quality control samples.
Transients useful for early detection of undesirable changes in process conditions appear as relatively high frequency xe2x80x9cspikesxe2x80x9d having a relatively short duration and relatively high amplitude when compared to the low frequency variation of the process equipment signals seen under normal operating conditions.
The monitored signals are not a direct measure of the chemical or physical parameters of a desired chemical reaction product by an analytical instrument, such as a direct measurement of particle size, or measurement a side stream of process material to determine viscosity. Rather, the signals typically are those normally available from the operation of the process equipment.
Transients present on downstream process equipment signals can be used to infer changes in process performance where the material parameter in question can not, or can not easily, be measured, or where a material characteristic can not be measured directly or promptly measured by an on-line product analyzer.
For example, transients appearing on a signal representing motor current for a piece of downstream rotating equipment can serve as an indication that the equipment is working against the introduction of inhomogeneous product (such as a polymer lump) from an upstream source. These relatively short term transients are often well tolerated by the process and control equipment they are related to, causing no apparent operational effect. Oftentimes the existence of such transients will not even be apparent until after a deliberate effort is made to sense or observe the transients. Once the presence of such transients is identified, further investigation will show a correlation to product inhomogeneity, and the signals can then be monitored in accordance with the invention to provide early indication of the need for upstream process control.
In the foregoing example, our invention is a very sensitive and timely method to detect the presence of strings and lumps, because these larger particles pass through the reactor and often through the extruder with no observable effect until they become very large. Nevertheless, examination of the powder feeder motor current signal showed the presence of small spikes believed to be caused by small lumps lodging in a relatively small gap between a rotating feeder blade and its housing. This early indication of deteriorating process conditions allows control action to be taken at an early point in time, where the required change in control can be less severe and more easily accomplished.
In a first embodiment of our invention, we disclose a method for controlling a chemical manufacturing process in which we first monitor a signal associated with downstream process equipment to detect transients present in the signal. We then infer a change in product quality associated with the detected transients, and adjust an upstream equipment process parameter in response to the inferred change in product quality.
While the invention can be used to control upstream equipment when the material whose quality is being controlled is either a solid or a viscoelastic fluid, such as a polymer melt, the invention is particularly useful in connection with the manufacture of polymeric powders such as polypropylene and polyethylene. Upstream equipment process parameters that can be varied include catalyst feed, cocatalyst feed, electron donor feed, monomer feed, hydrogen feed, comonomer feed, catalyst to cocatalyst ratio, catalyst activity control agent feed, reactor quench flow, reactor powder inventory, reactor temperature and pressure, whether controlled directly or indirectly by altering other parameters, reactor stirring or fluidization, or by altering combinations of the foregoing.
The invention is particularly useful for providing indications of agglomerated powders when the signal monitored is indicative of the performance of powder handling equipment such as rotary powder feeders.
Preferred embodiments of the invention monitor electrical current, voltage or frequency signals, hydraulic pressure signals or pneumatic pressure signals, as these signals frequently will exhibit transients of the type that can be correlated to subtle changes in product quality.
As used in this application, the term xe2x80x9cdownstream process equipmentxe2x80x9d means equipment located downstream of a chemical reactor which is used in the production or finishing of a chemical product, including, for example, motorized equipment such as pumps, conveyors, feeders, extruders and the like, but excluding equipment designed for the direct measurement of physical or chemical parameters, such as gas chromatographs, on-line spectroscopy equipment, or side stream viscosity or melt flow analyzers. This definition does not, however, preclude the use of inferential information from a process analyzer, for example such as from the motor controller circuit of a pump used to provide sample to an in-line viscometer.
A xe2x80x9csignal associated with downstream process equipmentxe2x80x9d can be any signal used to control or monitor the equipment. Thus, the term includes both controlled signals, such as motor current signals, or signals indicative of equipment condition, such as motor temperature, or equipment pressure or temperature.
A xe2x80x9cprocess parameter associated with upstream equipmentxe2x80x9d means any parameter useful for controlling a piece of process equipment located at a point earlier (xe2x80x9cupstreamxe2x80x9d) in the chemical manufacturing process to control the quality or quantity of material produced at a point located before the downstream process equipment. Examples of such parameters are the control of flow of feedstocks or catalyst to a reactor, or control of cooling water to an upstream heat exchanger. Other examples will be readily apparent to those skilled in the chemical manufacturing art.
In another embodiment of our invention, the motor current of a rotary feeder or other powder-handling equipment located downstream of a polymerization reactor is monitored for transients that indicate the presence of particles of unusually large shape or size. The magnitude and frequency of these transients can be compared over time to determine the presence of nonhomogeneous reactor effluent that otherwise is difficult to detect. Corrective action can then be taken to minimize the production of the undesired material.
As used herein, the term xe2x80x9cpowder-handling equipmentxe2x80x9d means any equipment used to transport, treat or operate a powdered material. As such, powder-handling equipment includes, but is not limited to, powder-transfer lines and valves such as in blow case systems, powder feeders or conveyers, extruders and purge columns.